Ozone and precursor trends can be used to measure the effectiveness of regulatory programs that have been implemented. In this paper, we review trends in the concentrations of O 3 , NO x , and HCs over North America that have been reported in the literature. Although most existing trend studies are confounded by meteorological variability, both the raw data trends and the trends adjusted for meteorology collectively indicate a general decreasing trend in O 3 concentrations in most areas of the United IMPLICATIONS Despite the implementation of many emission control programs during the past three decades, ambient O 3 levels in some parts of the United States, Canada, and Mexico continue to exceed the National Ambient Air Quality Standard or the Air Quality Objective set forth for O 3 . Detecting and tracking changes in ambient O 3 levels is complicated because of the strong influence of meteorological fluctuations on O 3 concentrations. The attribution of changes in O 3 to specific emission control plans that have been implemented is even more difficult due to limitations imposed by the quality and quantity of available data on O 3 and its precursors and uncertainties in emission estimates. This paper reviews the trends in O 3 and precursor concentrations reported over North America.